This invention relates to an automatic focusing device for a camera which is capable of adjusting the lens to a position corresponding to an object distance by means of a focus detection signal detected by an automatic focusing detecting device which automatically detects the focus position by scanning the range utilizing the principle of the coincident type range finder.
The automatic focus detecting device has a stationary mirror and a movable mirror. During the course of the scanning operation by the movable mirror, a peak correlative signal is produced when the optical information transmitted through the stationary mirror and the optical information transmitted by the movable mirror coincide. The angle of rotation of the movable mirror with respect to the direction of the incident light corresponds to the object distance. There have heretofore been introduced various methods of adjusting the lens to a fixed position employing the focus detection signal produced through procedure as hereinbefore described. In one of the conventional methods, a potentiometer is coupled with a movable mirror which performs a scanning operation from a minimum range through an infinite range, the resistance of the potentiometer when a peak correlative signal is produced during the scanning operation is memorized by a condenser or the like and the movable mirror and the potentiometer are uncoupled at the completion of the scanning operation of the movable mirror. The potentiometer is then coupled with an actuating member which drives the potentiometer in the reverse direction and the movement of the actuating member is interrupted when the potentiometer provides a resistance equivalent to the previously memorized resistance, then the lens is adjusted to a position corresponding to the position of the actuating member.
According to this conventional method, the movable mirror is required to rotate only in one direction during the scanning operation, furthermore, a single potentiometer serves both for range scanning and for range setting, thus providing considerably improved automatic focusing.
However, the method requires a reciprocating motion of the potentiometer requiring a sophisticated mechanism, and moreover, the durability and the stepping resistance of the potentiometer cause extra problems requiring high reliability.
In addition to those structural problems, stepless range adjustment is impossible by the actuating member due to mechanical restrictions resulting in stepped range adjusting. Consequently, stepless and accurate focus detection by the scanning operation of the movable mirror results finally in the stepped lens adjustment, therefore, analogue range adjustment is not always advisable.